Ceramic impedance elements and especially those having positive temperature coefficients of resistivity (PTC) have found wide application in many electrical devices. The unique property of having a fairly consistent resistance until the element reaches a predetermined temperature and of then very rapidly increasing in resistance for small additional temperature rises is very useful in various temperature sensing and thermally responsive switch applications. Such ceramic resistive elements are available from several commercial sources.
In a typical example of such a commercially available ceramic type resistive element, the element is formed of a PTC material such as lanthanum-doped barium titanate. This PTC material is generally made into disc-shaped resistive pills which have metallized coatings applied to opposite lateral surfaces thereof to provide conductive surfaces to which electrical contact can be made for directing electrical current through the pills. A continuing problem with commercially available pills of this type is that the thickness of the pills will vary from pill to pill. This fact makes it difficult to find a housing and contacting system which can accommodate the tolerance differences that occur when using the commercially available pills. Another problem is that the metallized coatings on the commercially available pills tend to have wide variations in the thickness of the coating and in the coatings' resistance to wear. With these problems it is difficult to find a suitable economical housing and contacting system for the resistive elements.
It is an object of the present invention to provide an improved impedance device incorporating ceramic impedance elements and to provide such a device with an improved housing and contacting system. It is a further object of the invention to provide for a system with a long life. It is still a further object to provide for a system which is economical to manufacture and is readily adaptable to mass production. Other objects will be in part apparent and in part pointed out hereafter.
The above objects are achieved by the invention by using a ceramic or other insulating material case which houses a ceramic impedance element such as a ceramic thermistor element that can be made up of one or more ceramic PTC pills between two resilient spring contacts. The use of these two resilient spring contacts allow the individual pill size to vary within reasonable tolerances and yet still be able to make contact with the spring contacts. This invention also calls for the use of a thin metallic disc having the same surface configuration as the ceramic thermistor element and which is inserted between the metallized surface of the element and the resilient sping contact. This metallic disc is needed because, when a voltage is applied across the ceramic element, the element heats up thus causing expansion to occur which, in turn, causes the resilient spring contact to become more compressed and to laterally rub against the metallized surface of the ceramic element. This rubbing action begins to wear through the metallized surface of the thermistor element and therefore to destroy the electrical conductive path between it and the resilient spring contact. The metallic disc exhibits a low contact resistance so as to not greatly increase the electrical resistance of the device while providing a uniform contacting surface between the metallized surface of the ceramic element and the resilient spring contact. The life of the device is therefore greatly increased because the point defects in the metallized surface due to rubbing and galling are greatly reduced.
This invention accordingly comprises the elements and combination of elements, features of construction and arrangement of parts which will be exemplified in the structures hereinafter described and the scope of the application of which will be indicated in the appended claims.